Page 38 - Ship Construction.DJ Eyres 6Ed

Ch03-H8070.fm Page 27 Wednesday, October 18, 2006 6:52 AM
Development of Ship Types 27
The requirements of the International Convention for the Prevention of
Pollution from Ships 1973 (see Chapter 29) and particularly its Protocol of
1978 have greatly influenced the arrangement of the cargo spaces of oil
tankers. A major feature of the MARPOL Convention and its Protocol has
been the provision in larger tankers of clean water ballast capacity. Whilst
primarily intended to reduce the pollution risk, the fitting of segregated
water ballast tanks in the midship region aids the reduction of the still water
--- ใช้เพื่อการศึกษาเท่านั้น---
bending moment when the tanker is fully loaded. It also reduces corrosion
problems associated with tank spaces which are subject to alternate oil and
sea water ballast cargoes.
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
In March 1989 the tanker Exxon Valdez, which complied fully with the then
current MARPOL requirements, ran aground and discharged 11 million gal-
lons of crude oil into the pristine waters of Prince William Sound in Alaska.
The subsequent public outcry led to the United States Congress passing the
Oil Pollution Act 1990 (OPA 90). This unilateral action by the United
States Government made it a requirement that existing single hull oil tankers
operating in United States waters were to be phased out by an early date, after
which all oil tankers were to have a double hull (see Figures 3.6 and 22.7).
In November 1990 the USA suggested that the MARPOL Convention
should be amended to make double hulls compulsory for new tankers. A
ึ
number of other IMO member states suggested that alternative designs
offering equivalent protection against accidental oil spills should be accepted.
In particular Japan proposed an alternative, the mid-deck tanker. This
design has side ballast tanks providing protection against collision but no
double bottom. The cargo tank space (see Figure 3.6) had a structural deck
running its full length at about 0.25 to 0.5 the depth from the bottom which
ensures that should the bottom be ruptured the upward pressure exerted by
the sea would prevent most of the oil from escaping into the sea.
In 1992 IMO adopted amendments to MARPOL which required tankers
of 5000 tonnes deadweight and above contracted for after July 1993, or
which commenced construction after January 1994, to be of double – hulled
or mid-deck construction , or of other design offering equivalent protection
against oil pollution. Existing tankers with single hulls without segregated
ballast tanks with protective location were to be phased out by June 2007.
Those with segregated ballast tanks with protective location, were to be
phased out by July 2021.
Studies by IMO and the US National Academy of Sciences confirmed the
effectiveness of the double hull in preventing oil spills caused by grounding
and collision where the inner hull is not breached. The mid-deck tanker was
shown to have more favourable outflow performance in extreme accidents
where the inner hull is breached. The United States authorities considered
grounding the most prevalent type of accident in their waters and believed
only the double hull type prevented spills from tanker groundings in all but
the most severe incidents. Thus, whilst MARPOL provided for the acceptance